Prevention of scale formation in uranium solvent extractor



Nov. 5, 1957 J. W. DELAPLAINE PREVENTION OF SCALE FORMATION IN URANIUMSOLVENT EXTRACTOR Filed Nov. 16, 1955 DIGEST SLURRY FEED TANK EXTRACTSCRUB WATER EXTRACTOR/V RAFF|NATE INVENTOR. JOHN W- DELAPLAINE ATTORNEYUnited States Pat fi'tn 2,812,232 PREVENTION oF SCALE I'ioRMA-TIjoN is:

SOLVENT EXTRACTOR John W; Delap laine," l hilailelphia, 11 a,, assignor,niesne assignments, to the United Statesof America as representedby theUnited states Ato'n'ii Eiirgy'c miifiss'ion Application November 16,1953,"SerialNo.' 392,530

9Cii1'iii1js'. (Ci. 23-145 The present invention? relates to a "thod forpr 1 ing scale formation in uraniuffi solv'e'rit eiitractio ment, andmore particularly s'cal'e fo'r'inationc'aused by the precipitation ofsilica and calciurn andlead siilfates.

In the recovery of uranium from "natural: ores can ir'i ing the same,the usualprocedure is to digs'fthe ore" with nitric acid to produce anaqueous acidslurry. The-slhrry, for instance one obtained by digestionof pitchblende, contain in addition to salts turaniu'm; salts ofcalcium, lead, barium and generally also s'ome salts or various othermetals. Uranium is separated from the slurry b'y aprocedur e involvingselectiveextraction, usually by means of mixed organic solvents.Although not limited thereto, one form of apparatus largely employed forsuch'e'xtraetion is a pulse column. I V a One of the major difficultiesencountered ijn's' "ent" traction processes employed in the productionoff am metal from ores 'c'ontaininglead calcium andvarious s'filffides'is the large amount of 'scaIe formation that occurs in theextraction'eqnipriient', especially inside the solveiit extractioncolumn. This seale formation is ttie 'ause of frequent equipmentshutdowns for clean-out'pur oses.

The principal components of this scale Were-'fonndto be silica, calciumsulfate and lead sulfate. Silica appears to precipitategradlially fromthe aqueous feed material charged to the extraction equipment;and'it-is" concluded that the feed material is supersaturatedwith-"silica vvhen fed to the extraction system. Calcium" sulfate'andlead sulfate, on th'eother hand, appear to precipitate as uranylnitrate is extracted from the aqueousfe'ed material by the organicsolvent. Barium -and radiumsulfates also appear to precipitate fromsolution' together with tlie lead-and calcium sulfates. The resultingscale is difiicult td'r'e move from-the interiors and: interna-ls ofithe extraetioh equipment and is extremely radioactive.

It is an obj ect of the present invention toprovide' an effective methodfor reventing seale' tonnatio'neir the interior walls and internalmembers of solventeiitractioii equipmentwliile employed foi tlieseleetiv solvent 'eXtiaetion of uranium from aqueous feed materials.

Other objects will become apparent from a reading of the followingdescription of 1 a particular er'nb'oditnient of the invention, which isgiven"vi/ithdeference"ttflliealceorii panying drawing, illustrating thextractiomprocess sclie matically by flow diagram. I

Digest slurry, such as'that produced by the nitriw'acid digestion ofpitchblende ore, is continuously Withdrawn 2,812,232 -Patented Nov. 5, 1957 r i Z 'liottoiii thereof, throughline 13 atga predetermined' rate.Ari other solvent that is not attacked by H.NO3 and, has sf fac v x rapQY im u e ti nrroperti be used. For aslurry having a concentration; ofabout 2 grams uranium per liter, the ratio of org-ariicsolvent scendsfrom" their)? to th e bottom of extractor 11.

A continuous 'countercurrent solvent extraction takes 151m iii extractorl1, and substantially allot the uranyl n1 ate is egrtracted'directlyfrom both the liquid and solids components ofth e digest slurry by the'organic' solvent. The organic eiiit racfipontaining the uranylnitraterlargely ii the form of a uranyl; nitratetributyl phosphate com-915x; is continuously withdrawn from the top of extractor 1111 througline 14, and is introduced into a scrubber (not shows) vvliere isscrubbed with water to: removeyery small amounts of residual impurities,and processed by tiableik riov vn method to produce anaqueous uranyl nate product. The used vvater produced in thisscrub- Bing opei ation iscalled scrub water. A stream of rah 1naYe slurry, substantially free ofuranium, is continuously withdrawnfrorn tliebottorn of colurnn 11through line 15.

Iii fireyiious" known methods; as the uranyl nitrate' is eYtracted-fromthe aqueousphaseby the organic solvent ek t'ractorl l l, calcium, lead;radium and bariumsul fates pfecipitateiaridtend to deposit as scale onthe inte o'rs'urfaees of extractor 11 and-interconnecting piping. T sscale is very" difficult to remove, and is extremely i ii daetiv' V I, Av Scale formation infeXtractor l1 5. PY ntQd in the r'xithodlof tl i'e'present invention by (l) preventing super.- s if b f he f slurry wit i ad eal sulr rates, Q2) controlling the concentration of uranyl nitrate edslur-fiat orj'above about 200 grains periliter V iiti'olof igestionconditions, (3) controlling the concentration df iii'aiiyl nitrate inthe organic phase ieaviii extra'tern at'aboiit' 65 to a outss' pertainiiq rabd about 1016 about 8O percent of the saturation (4); controllingthe" nitric acid concentration in the aqn' as feed material and the acidgradieiitiji the aqua, t' proceeds through extractor 11, and (5 y theslurry inside the column with" an aqueous diluent such as scrub vv'ater.

o y Way of l'anation, if the solubility ofla c coir i'poinid decreas ina' solution vvhich N saturated, the soliitioii becomes supersaturatedandLSal- \fi/lietlir-zr or notscaling does ig-gu is the d li'ee oflsuper saturation and the rate liigh b "au" solutions 2 acid but alsowith uranyl nitrate. In the extractor y nitrate" is removed from theaheous solution andsupplied to the organicsolutioh. Thiscausestlre'solubility' of calciiim'sulfate" and lead sulfate-and similarniaterials td decreaseybecause of theremoval of thesaltin'g-in eflectSince uranium is1rem'9Vdjcon-' tinuo usly as the slurry passes downwardthrough the column,-the solubility of these'salts also decreases conuqud V, v. i scd l ii a th e ln niss nallyfi t atsd 6r s i ht su e sa ur edths iunsv i t 9 L a j sillfate, some tec hnique must be utilized toofiset this or the; presence or the nitrate 1on dfigiflateS Iibf dillyWith the lfi solution is unsaturated, the removal of the salting-inmaterial may never cause the solution to become supersaturated. If thefeed solution is high in calcium and low in lead only afew of the aboveproposed techniques may be required. For example, with a feedunsaturated in lead sulfate, it may not be necessary to add nitric acidto the organic solvent to produce a high nitric acid concentrationgradient in the extractor. This is so because the slurry will not becomesupersaturated with lead sulfate at any point in the column. If,however, the feed solution is saturated with lead sulfate, acidulationof the organic solvent is required.

The method of the present invention presents the combination oftechniques required for increasing the solubility of the calcium or leadsulfate materials, or decreasing the concentration of the ionsthemselves so that the solubilities of these salts will not be exceeded.This combination of techniques has been developed for a feed saturatedin both calcium sulfate and lead sulfate. Dilution of the calcium, leadand sulfate ions is brought about by the introduction of any suitablediluting material. The amount of dilution required, if the aqueous washproduct from the scrub column is used for dilution purposes, isdetermined by a complete material balance around both the extractioncolumn and the scrub column. It is desirable to utilize an aqueousslurry feed rich in uranium because the material balance for about 80percent uranium saturation of the organic phase equals a value such thatthe aqueous phase in the extraction column is never supersaturated withcalcium or lead sulfate.

Supersaturation of the feed slurry with silica and metal sulfatesisprevented as follows. Th feed slurry is preheated to a temperature ofabout 150 to 250 F, preferably about 200 F. The hot slurry is thencooled to atmospheric temperature in tank 10, or in a separate vessel,while it is continuously stirred to promotecrystallization andprecipitation of the dissolved silica and metal sulfates.Supersaturation of the feed slurry with silica and metal sulfates isprevented in the cooled feed slurry by this operation. Since thesolubilities of the silica and metal sulfates in the feed slurrydecrease with increasing temperature, a maximum amount of silica andmetal sulfates is precipitated from the feed slurry, and a minimumamount of dissolved silica and metal sulfates enters extractor 11 in thefeed slurry. In instances where the silica and calcium and lead sulfatecontents of the feed are relatively low, the foregoing step may beomitted.

The concentration of uranyl nitrate in the feed is maintained aboveabout 200 grams per liter by controllingdi gestion conditions. For aconcentrated uranium-ore-this is a relatively simple matter. Theconcentration can be obtained by digestion with 50 to 60 percent nitricacid. For a dilute uranium ore more concentrated acid, or concentrationby boildown may be required. The concentration of uranyl nitrate in theorganic extract is controlled at a desired level below the saturationpoint by controlling the organic to aqueous volumetric ratio in thecolumn. Thus the product concentration of uranium in the extract is setby a material balance on the extraction system, as pointed out above,and the preferred concentration is about 70 to about 80 percent ofsaturation.

Control of the nitric acid concentration in the aqueous or rafiinatephase, as it proceeds through extractor 11, is highly desirable toreduce the scaling tendency of the raflinate phase during the solventextraction operation. It has been found desirable to increase the nitricacid concentration as the aqueous phase descends from the top to thebottom of column 11. It has also been discovered that the desired nitricacid concentration in the aqueous phase should be about 3.0 to 3.5normal at the supply level of the slurry. This concentration ispreferably achieved by continuously saturating the organic solvent withnitric acid while the organic solvent is continuously charged to column11 through line 13.

Dilution of the aqueous phase is accomplished by continuouslyintroducing Water or dilute nitric acid into the top of extractor 11through line 16 at predetermined rates to dilute the feed slurry withinthe column. The dilute nitric acid used for this dilution operation canbe aqueous scrub water obtained as described above from a scrubber (notshown). This scrub liquid may contain about to 200 grams nitric acid perliter and about 50 to about 200 grams per liter of uranium as uranylnitrate. The composition of the scrub water used depends upon thetemperature of the scrubbing operation, thecomposition of the organicextract, and the ratio of organic phase to aqueous phase in the scrubcolumn. The amount of water or acid-water added to the feed slurry ispreferably about 30 percent by volume of the slurry.

In the feed slurry charged to extractor 11, calcium, lead, .radium, andbarium sulfates are retained in solution, i. e., .salted in by uraniumnitrate and by nitric acid dissolved in the feed. As the uranyl nitrateis extracted from the feed slurry by the organic solvent in column 11,these dissolved sulfates may exceed their solubility limits and, unlesssuitable preventive measures are taken, the sulfates deposit as scale onthe interior surfaces in extractor 11.

For efficient operation, it is desirable to charge a feed slurry toextractor 11 which has a maximum uranyl nitrate concentration. Inaddition, it is desirable to maintain the uranyl nitrate concentrationabove about 200 grams uranium per liter to minimize scaling tendencies.The free nitric acid concentration is maintained at about 3.0 to 3.5normal in the aqueous phase at the supply level in extractor 11 becauseat higher nitric acid concentrations lead sulfate is precipitated. Thisprecipitation occurs because of the high concentrations of Pb++ ion andSO4= ion in the feed slurry charged to extractor 11. In other words, theprecipitating effect of uranyl nitrate removal on lead sulfate is higherat higher nitric acid normalities. At nitric acid concentrations below3.0 normal, on the other hand, calcium sulfate is precipitated becauseof the extremely large decreases in the solubility of calcium sulfate asuranyl nitrate is extracted from the aqueous phase in extractor 11 bythe organic solvent. At a 3.0 to 3.5 normal nitric acid concentration inthe aqueous phase, the change in the solubility of calcium sulfate for agiven decrease in the uranyl nitrate concentration in the aqueous phase,is much less than at nitric acid concentrations below about 3.0 normal.

In the known previously used process, the scaling tendency of the feedslurry is reduced by the addition thereto of a barium salt solution toprecipitate sulfate as BaSO4, and reduce the S04 ion concentration tothe point where the solubilities of the other dissolved metal sulfatesare not greatly exceeded. The disadvantages of this method are: 1) highchemicals cost; (2) considerable scaling occurs in spite of the bariumaddition; and (3) high cost of filtration equipment to remove theprecipitated sulfates.

In the method of the present invention, dilution takes place inthe topof column 11 by the normal recirculation of scrub water, as pointed outabove. Thus, the con- -centrations of the dissolved metal sulfatesentering column 11 is .unatfected prior to charging the feed slurry toextractor 11.

The ionic concentrations in the aqueous feed, e. g., of calcium andsulfate, are decreased by'the dilution mentioned above in proportion tothe amount of this dilution; Forexample, a 30 percent dilution withscrub Water reduces the product of these two concentrations to (0.7)(0.7) or 49 percent of the original value. Consequently, up to a 1percent reduction in the solubility product of calcium sulfate due tothe precipitating effect of uranium removal can be tolerated. In themethod of the present invention, as applied to feed materials saturatedor supersaturated with calcium, lead. and sulfate-ions, control of thefeed composition, level of' saturation, preheatingconditions and acidityof the solvent are maintained at such values; that the solubilityproduct constants of the dissolved sulfates are never exceeded inthesystem. If the uranium concentration in the feed drops below about200 grams per liter of uranium, and thefeed contains enough dissolvedcalcium or lead sulfate. to saturate it, the decrease-in the solubilityproducts in theaqueous phasein column 11 will be greater than about. 50percentand precipitation may occur. i r

In actual practice, the amount. of aqueous phase dilu; tion required incolumn 11' is a functionofi the amount of scrub water used, the degreeof uranium saturation achieved in the organic solvent leaving extractorcolumn 11; and the uranium content of the' feed charged to column 11.The metal: sulfate dilutionachieved in the aqueous phase is proportionalto the, volume of scrub water used per volume. of slurry feed; A fixed:ratio of scrub water to organic extract isrequired in,-a particularapplication of the process to obtain the. desired purification of theorganic extract in the scrub column. The metal sulfate dilution obtainedin practice is proportional, to the volume of organic extract producedper; volume of slurryfeed employed. Since the uraniumis essentiallycompletely extracted from the feed the Weight rate of uranium enteringthe column must, equal, the weight rate ofuranium leaving the column.Thus, for a given uranium saturation of the organic extract, thevolumeof organic extract per volume of feed, and hence the aqueousphasedilution is directly'proportional to the concentration of uranium in theslurry feed. Similarly, for a given uranium concentration in the feed,the lower the organic extract uranium concentration is below thesaturation value, the greater the aqueous phase dilution achieved.However, high uranium saturation is required in the. organic extractphase for the desired product purity. Therefore, a balance is struck atabout 65 to about 95 percent and preferably about 70 to about 80 percenturanium saturation in the organic extract.

Scaling is reduced in the method of the invention by increasing the acidconcentration of the aqueous. phase passing from the top to the bottomof column 11. This increase is obtained by acidifying the organicsolvent entering column 1 1 through line 13. As uranium is extractedfrom the aqueous feed by the organic phase, the nitric acidis forced outof the organic phase. This occurs because the amount of tributylphosphate in the organic solvent is insufiicient to complex orassociatewith both uranyl nitrate andnitric acid, and complexespreferentially with the uranyl nitrate. By saturating the organic phaseentering column 1 1 with nitric acid, or partially saturating it at alevel of about 70 to about 80 percent, and running at a uraniumsaturation of about 75 to 80 percent in the aqueous feed to column 11,the acid-concentration of the aqueous product or raflinate; withdrawnfrom column 11, through line 15, can be made to be higher in. nitricacid concentration by about 1.0 to 1.5 normal than the nitric acidconcentration of the aqueous feed. The beneficial effect on lead sulfateisshown in the table below:

Solubility Product of The required increase of nitric acid is also afunction'ot' the concentration of uraniumin the aqueous phase or thepercent-of saturation oi -the uraniumill-i116 exit aqueous 6 phaseonraffinate. The higher theuranium concentration, the greater the acidconcentration. in theraflinate. As stat edrpreviously, the percentsaturation is balanced by all: considerations at about to 80. percent inthe aqueous phase.

Scaling can also be reduced by adding ferric nitrate to the scrubiwater.This increases the solubility of the dissolved'sulfates substantiallyand1preventstheir precipitation. Calgon (sodium hexametaphosphate) mayalso be added to minimize sulfate deposition. The use of'either of theseexpedien'ts is expensive, and is not necessary if the. factorspresented-in the description above are controlled within the properlimits.

Obviously there; are other applications and modifications. of the methodof the present invention than those mentioned above. It is to beunderstood, therefore, that the scope of this invention isto' bedetermined only as required by the following claims, when construed inthe light of the prior art.

What-is claimed is: a

l- A method of preventing scale formation during selective solventextraction of uranyl nitrate from an aqueous feed material whichcomprises preheating the feed material to a temperature in therange ofabout 150 to 250 F., cooling thehot feed material to atmospherictemperature. While continuously agitating the slurry to promotecrystallization and precipitation of silica and metal salts, controllingthe uranium concentration of the feed material above about 200 grams perliter, charging the, feed material into an extractionlzone intointimatecontact with an organic selective solvent, adiustingthe concentration ofuranyl nitrate in the organic phase discharging from the extraction zoneto about 65 to, about 95'percent' ofthe saturation. value by controllingthe organic to aqueous phase ratio in the extraction zone, andmaintaining an, increasing nitric. acidconcentration gradient in theaqueous phase irrthe extraction zone by partially saturating the organicselective solvent entering said zone with nitric aeidata level of about70 to about 2; A method of preventing scale formation during selectivesolvent extraction, of uranyl nitrate from. an aqueous feed materialwhich comprises preheating the feedmaterial to atemperature in therangev of about 150 to 250 F., cooling the hot, feedmaterialtoatmospheric temperature while continuously, agitating theslurry to promote crystallization and, precipitation, of silica andmetalsalts, controlling thev uranium concentration of the feed materialabove about 200 grams per liter,controlling.

the nitric acid. concentration ofsaid feed material at about 3.0 to 3.5normal, charging the feedmaterial into an extraction zone into intimatecontact With anorganic-selective solvent, adjusting the concentration ofuranyl nitrate in th. organ ic phase dischargingfrom the extraction zoneto about 65 to about percent of the saturation value by controlling theorganic to aqueous phase ratio in the extraction zone, and maintaininganincreasing nitric acid concentration gradient inrthe aqueousphase in theextraction zone bysaturatinlg, the. organic selective solvent Withnitric acidat a level of about 7.0 to -about,80 percent.

3, A method of preventing scale formation during selective. solventextraction of uranyl nitrate from an aqueous feedHmateria-l whichcomprises preheating the feed material to a temperature in the range ofabout 2150? to 250 -F., cooling the hot feed, material toatmospherictemperature. While continuously agitating the, slurry to promotecrystallization and precipitation of. silica and metal salts,controlling the uranium concentration of the feed material above about200 grams per liter, changing the feed material into an extraction zoneinto initimate contact with, an organic selective solvent, adjusting theconcentration of'uranyl nitrate in the organic phase discharging fromthe extraction zone to about 65 to about 95 percent of thesaturationvalue by controlling the organic toaqueous phase rati'oin theextraction zone, and

*maintaining the nitric acid-concentration in the aqueous ",7 phasedischarging from the extraction zone greater-by 1.0 to 1.5 normal thanthe nitric acid concentration of the aqueous feed material by saturatingthe organic selective solvent entering said zone with nitric acid at alevel of about 70 to 80%.

4. A method of preventing scale formation during continuous selectivesolvent extraction of uranyl nitrate from an aqueous feed material whichcomprises preheating the feed material to a temperature in the range ofabout 150 to 250 F., cooling the hot feed material to atmospherictemperature while continuously agitating the slurry to promotecrystallization and precipitation of silica and metal sulfates,continuously controlling the uranium concentration of the feed materialabove about 200 grams per liter, continuously charging the feed materialinto a continuous solvent extraction column for intimate contractingwith an organic selective solvent, continuously maintaining theconcentration of'uranyl nitrate in the organic phase discharging fromthe extraction column at about 70 to about 80 percent of the uranylnitrate saturation value by controlling the organic to aqueous phasevolumetric ratio in the extraction column, continuously diluting theaqueous phase inside the extraction column with an aqueous diluent, andmaintaining a nitric acid concentration gradient in the aqueous phase inthe extraction column by adding nitric acid to the organic solventcharged to the extraction column to saturate said solvent with saidacid.

5. A method of preventing scale formation during continuous selectivesolvent extraction of uranyl nitrate from an aqueous feed material whichcomprises forming a nitric acid digest slurry by the nitric aciddigestion of pitchblende ore, preheating the feed material to atemperature in the range of about 150 to 250 F., cooling the hot feedmaterial to atmospheric temperature while continuously agitating theslurry to promote crystallization and precipitation of silica and metalsulfates, continueously controlling the uranium concentration of thefeed material aboveabout 200 grams per liter, continuously charging thefeed material into a continuous solvent extraction column for intimatecontacting with an organic selective solvent, continuously maintainingthe concentration of uranyl nitrate in the organic phase dischargingfrom the extraction column at about 70 to about 80 percent of the uranylnitrate saturation value by controlling the organic to aqueous phasevolumetric ratio in the extraction column, continuously diluting theaqueous phase inside the extraction column with an aqueous dilvent, andmaintaining a nitric acid concentration gradient in the aqueous phase inthe extraction column by adding nitric acid to the organic solventcharged to the ex traction column to saturate said solvent with saidacid.

6. A method of preventing scale formation during continuous selectivesolvent extraction of uranyl nitrate from an aqueous feed material whichcomprises preheating the feed material to a temperature in the range ofabout 150 to 250 F., cooling the hot feed material to atmospherictemperature while continuously agitating the slurry to promotecrystallization and precipitation of silica and metal sulfates,continuously controlling the uranium concentration of the feed materialabove about 200 grams per liter, continuously charging the feed materialinto a continuous solvent extraction column for intimate contacting withan organic selective solvent consisting of about 22.5 volume percenttributyl phosphate and about 77.5 volume percent kerosene, continuouslymaintaining the concentration of uranyl nitrate in the organic phasedischarging from the extraction column at about 70, to about 80 percentof the uranyl nitrate saturation value by controlling the organic toaqueous phase volumetric ratio in the extractioncolumn, continuouslydiluting the aqueous phase inside the extraction column with an aqueousdiluent, and maintaining a nitric acid concentration gradient in theaqueous phase in the extraction columntby adding nitric acid to theorganic solvent charged to-the extraction column to saturate saidsolvent with said acid,

7. A method of preventing scale formation during continuous selectivesolvent extraction of uranyl nitrate from an aqueous feed material whichcomprises perheating the feed material to a temperature in the range ofabout 150 to 250 F. cooling the hot feed material to atmospherictemperature while continuously agitating the slurry to promotecrystallization and precipitation of silica and metal sulfates,continuously controlling the uranium concentration of the feed materialabove about 200 grams per liter, continuously charging the feed materialinto a continuous solvent extraction column for intimate contacting withan organic selective solvent, controlling the nitric acid concentrationof the aqueous phase at the feed level in the extraction column at about3.0 to 3.5 normal, continuously maintaining the concentration of uranylnitrate in the organic phase discharging from the extraction column atabout 70 to about percent of the uranyl nitrate saturation value bycontrolling the organic to aqueous phase volumetric ratio in theextraction column, continuously diluting the aqueous phase inside theextraction column with an aqueous diluent and maintaining a nitric acidconcentration gradient in the aqueous phase in the extraction column byadding nitric acid to the onganic solvent charged to the extractioncolumn to saturate said solvent with said acid.

8. A method of preventing scale formation during continuous selectivesolvent extraction of uranyl nitrate from an aqueous feed material whichcomprises producing a nitric acid digest slurry by the nitric aciddigestion of pitchblende ore, preheating this feed material to atemperature in the range of about to 250 F., cooling the hot feedmaterial to atmospheric temperature while continuously agitating theslurry to promote crystallization and precipitation of silica and metalsulfates, continuously controlling the uranium concentration of the feedmaterial above about 200 grams per liter, continuously charging the feedmaterial into a continuous solvent extraction column for intimatecontacting with an organic selective solvent, said solvent consisting ofabout 22.5 volume percent tributyl phosphate and about 77.5 volumepercent kerosene, continuously maintaining the concentration of uranylnitrate in the organic phase discharging from the extraction column atabout 70 to about 80 percent of the uranyl nitrate saturation value bycontrolling the organic to aqueous phase volumetric ratio in theextraction column, continuously diluting the aqueous phase inside theextraction column with an aqueous diluent, and maintaining a nitric acidconcentration gradient in the aqueous phase in the extraction column byadding nitric acid to the organic solvent charged to the extractioncolumn to saturate said solvent with said acid.

9. A method of preventing scale formation during continuouscountercurrent selective solvent extraction of uranyl nitrate from anaqueous feed material which comprises preheating the aqueous feedmaterial to a temperature below its boiling point, cooling toatmospheric temperature while agitating the feed material to promotecrystallization and precipitation of silica and metal sulfates therein,feeding the aqueous feed material into an extraction column at a levelabove the middle of the column, continuously introducing an organicselective solvent into 'said column near the bottom thereof,continuously discharging a uranium-rich extract from the column near thetop thereof, continuously discharging a rafiinate substantially free ofuranium from the column near the bottom thereof, introducing scrub waterfrom a scrubbing operation of the uranium-rich extract into the columnat a level above that of the aqueous feed introduction, introducingnitric acid into the column at the level of introduction of the organicselective solvent, continuously maintaining the concentration of nitricacid in the organic phasenearithebottom'of the column at about 770 toabout 80 percent of the saturation value, maintaining the nitric 1.0 to1.5 normal above that of the aqueous phase at the acid concentration inthe aqueous phase at about 3.0 to level of aqueous feed materialintroduction.

3.5 normal at the level of aqueous feed introduction into the column,and maintaining the nitric acid concentration References Cited in thefile of this P in the aqueous phase near the bottom of the column at 5UNITED STATES PAT T 2,227,833 Hixson et a1. Jan. 7, 1941

1. A METHOD OF PREVENTING SCALE FORMATION DURING SELECTIVE SOLVENTEXTRACTION OF URANYL NITRATE FROM AN AQUEOUS FEED MATERIAL WHICHCOMPRISES PREHEATING THE FEED MATERIAL TO A TEMPERATURE IN THE RANGE OFABOUT 150* TO 250*F., COOLING THE HOT FEED MATERIAL TO ATMOSPHERICTEMPERATURE WHILE CONTINOUSLY AGITATING THE SLURRY TO PROMOTECRYSTALLIZATION AND PRECIPITATION OF SILICA AND METAL SALTS, CONTROLLINGTHE URANIUM CONCENTRATION OF THE FEED MATERIAL ABOVE ABOUT 200 GRAMS PERLITER, CHARGING THE FEED MATERIAL INTO AN EXTRACTION ZONE INTO INTIMATECONTACT WITH AN ORGANIC SELECTIVE SOLVENT, ADJUSTING THE CONCENTRATIONOF URANYL NITRATE IN THE ORGANIC PHASE DISCHARGING FROM THE EXTRACTIONZONE TO ABOUT 65 TO ABOUT 95 PERCENT OF THE SATURATION VALUE BYCONTROLLING THE ORGANIC TO AQUEOUS PHASE RATIO IN THE EXTRACTION ZONE,AND MAINTAINING AN INCREASING NITRIC ACID CONCENTRATION GRADIENT IN THEAQUEOUS PHASE IN THE EXTRACTION ZONE BY PARTIALLY SATURATING THE ORGANICSELECTIVE SOLVENT ENTERING SAID ZONE WITH NITRIC ACID AT A LEVEL OFABOUT 70 TO ABOUT 80%.